NICE, France -- The Islamic State on Saturday claimed the Tunisian man who barreled his truck into a crowd in the French resort city of Nice was a "soldier" of the group. It's the first claim of responsibility for an attack that claimed 84 lives at a July 14 fireworks display.

The claim -- circulated on social media by a news outlet affiliated with the group -- didn't name Mohamed Lahouaiej Bouhlel, the 31-year-old Tunisian who authorities say was behind the wheel as a truck crashed into a crowd of revelers at a fireworks display on Thursday. But the statement quoting a security member of the group said the man was following calls from IS to target nationals of countries fighting it.

The veracity of the group's claim couldn't immediately be determined, but what is known so far about Bouhlel thus far suggests a troubled, angry man with little interest in the group's ultra-puritanical brand of Islam.

It's also unclear whether or not he was acting alone. The Paris prosecutor's office said Saturday that five people are in custody following the attack.

The identities of most of those brought into custody were not clear. But neighbors in the Nice neighborhood where the Bouhlel used to live told The Associated his estranged wife had been taken away by police on Friday.

The claim of responsibility came as French security chiefs met in Paris and as Nice's seaside boulevard, the famous Promenade des Anglais, is slowly coming back to life. A makeshift memorial of bouquets, candles and messages had been set up near one end of the expansive avenue.

France is observing three days of national mourning in homage to the victims -- although that hasn't stopped politicians from sniping at each other over who bore responsibility for the failing to stop the attack.

In an open letter published on the Nice Matin newspaper's website, regional council President Christian Estrosi -- a member of France's opposition Republicans -- described the country's current leadership as "incapable." He said he had requested that the police presence be reinforced in Nice ahead of the display but was told there was no need.

France is heading into elections next year, and the deeply unpopular French President Francois Hollande is facing multiple challengers from within his Socialist Party, from the right-wing Republicans, as well as the far-right National Front.

Following the security meeting, Hollande met with ministers at the Elysee Palace.

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Sarah El Deeb in Beirut and Raphael Satter in Paris contributed to this report.

This article was written by The Associated Press and Colleen Barry from The Canadian Press and was legally licensed through the NewsCred publisher network.